The organic electroluminescent device, also called the organic light-emitting diode (OLED), is a device that converts electrical energy into light energy by applying voltage thereon, and an ideal display for cell phones and color televisions. The OLED display has advantages of broad viewing angle, significant energy saving, high luminescence efficiency, etc. The unique advantages of the OLED closely relate to carrier transport, luminescence and electrode materials adopted therein and device structures in which the luminescence material is a core component of the OLED and can be divided into two types of fluorescent materials and phosphorescent materials.
The green light phosphor material of organic luminescence is one phosphorescent material on which has been earlier researched; undoubtedly, iridium complexes have been widely researched in the phosphorescent material.
It is based on heavy metal atoms of strong spin-orbit coupling mixed with singlet and triplet of complex in heavy metal complexes, so that an originally forbidden transition from triplet to ground state can be overcome, and then luminescent efficiency of the materials are greatly increased. In comparison of the iridium complexes and the other heavy metal complexes, the iridium complexes are convenient to handle and adjust color, and thus have been more widely researched.
As early as in 1999, team Forrest of Princeton University U.S.A. doped a phosphorescent iridium complex Ir(ppy)3 in a host 4,4-N,N′-dicarbazole-biphenyl to obtain an OLED device having an external quantum efficiency of 8% and a power efficiency of 31 lm/w. Soon after, Ikai et al. further improved the host material and add a hole and exciton block layer to obtain a device doped with Ir(ppy)3 having an external quantum efficiency increased to 19.2% by applying voltage of 3.52V thereon. Such low-voltage electroluminescence and luminescent efficiency of 72 lm/W allow the OLED be a uniform scattering light source for next generation power-saving display and illumination. Such excellent performance especially comes from effectively transferring all singlet and triplet of the host material to the object Ir(ppy)3 so as to obtain a high external luminescent efficiency.
Then in 2000, team Forrest further doped Ir(ppy)3 in several host materials having electron transporting performance to fabricate devices that had external quantum efficiency up to (15.4±0.2)% and maximum power efficiency of (40±2) lm/W. In 2001, the team further reported a new green light phosphorescent material Ir(ppy)2acac, they doped the phosphorescent material in TAZ to fabricate an organic electroluminescent device that had a maximum external up to (19.0±1.0)% and power efficiency up to (60±5) lm/W; based on calculation, an internal quantum efficiency thereof is (87±7)%, almost 100%, these fully shows prospect of phosphorescent material in application of electroluminescent field.
Such complex containing 2-phenylpyridine (ppy) as ligand has simple structures and excellent performance. Many research teams still try to discover synthesis of phosphorescent complex based on ppy and to research device performance.
Kim et al. reported a series of green light materials based on 2-phenylpyridine, they introduced methyl formed at different locations of phenyl and pyridine to synthesize 5 high efficiency green light materials in which have a maximum quantum efficiency of 52%; additionally, they further introduced some other large steric hindrance groups to synthesize some green light materials of novel structures, organic electroluminescent devices fabricated with the materials has a maximum external quantum efficiency of 25.6% and electric current efficiency up to 84.4 cd/A. Gao et al. synthesized a novel ligand BPPya based on change of 2-phenylpyridine and used the novel ligand to fabricate iridium complex Ir(BPPya)3 having good luminescent efficiency and thermal stability. Since energy levels of object and subject material are close, an electroluminescent device based on the complex effectively reduces triplet-triplet quenching so as to have a maximum external quantum efficiency up to 14.6%, electric current efficiency of 52 cd/A and power efficiency of 33.5 lm/W. In 2013, He et al. synthesized a green light iridium complex; they used tfmppy as main ligand and tpip as auxiliary ligand, the synthesized complex has an emitting peak at 520 nm, a device fabricated by using high triplet level mCP as host material has an external quantum efficiency up to 20.8% and power efficiency up to 66.3 lm/W.
In recent years, with development of OLED technology, more and more high efficiency green light materials have been developed, and performance of recent green light OLED devices are getting better due to new technologies adopted to increase light extraction efficiency (such as light extraction technology, etc.)
In 2011, Helander et al., Toronto University reported the conventional ITO anode substituted with a chlorinated ITO anode, without hole injection and transport material, using Ir(ppy)2acac as light emitting material and adopting light extraction technology at the same time to fabricate green light devices that obtain an external quantum efficiency up to 54% and a power efficiency of 230 lm/W. Recently, Li et al., U.S.A reported a phosphorescent device structure using single-layer grapheme as anode and Ir(ppy)2acac still as object material, it allows holes from the single-layer grapheme directly inject into light emitting layer, the device has very excellent performance and an external quantum efficiency up to 60% by adopting the light extraction technology or 45% even at brightness of 10000 cd/m2, and a color rending index (CRI) of the device is 85.
Although structures of the reported complexes based on 2-phenylpyridine are simple and have great advantages from the perspective of material synthesis, the complexes do not contain modified units of special functions; therefore, it is needed to modify device structures, due the materials do not have the special functions such as means of introducing a special carrier function layer or adopting complicated light extraction technologies, that allow the structures and art of the devices become cumbersome and complicated. On the other hand, for simplifying the organic electroluminescent device structure and fabrication arts, some special units can be introduced to the molecules such as dendrimers; however, it is undoubted to allow the material synthesis and preparation become cumbersome and complicated.